The goal of this Phase II SBlR project is to develop clinically useful inhibitors of the multidrug-efflux transporters of the Gram positive pathogens, Staphylococcus aureus and Streptococcus pneumoniae. These transporters are partially responsible for the intrinsic and acquired resistance of these bacteria to fluoroquinolone antibiotics. Preliminary results demonstrate that inhibition of the multidrug transporters would dramatically increase the effectiveness of fluoroquinolone therapy by both increasing the intrinsic susceptibility of these pathogens to fluoroquinolones and suppressing the emergence of drug-resistant variants. In the ongoing Phase I SBIR project we screened a library of synthetic chemicals and identified several very promising lead compounds that effectively inhibit the S. aureus multidrug transporter NorA and the presently unidentified multidrug transporter of pneumoniae. Here we propose to optimize the properties of lead inhibitors through several cycles of improvement, each involving synthesis of structural analogs of the leads, biological evaluation of their activity and toxicity, and QSAR analysis. The improved inhibitors will be thoroughly characterized, including preliminary toxicology studies. The obtained results will eventually allow us, in alliance with a large pharmaceutical company, to develop a potent combination antimicrobial drug containing a fluoroquinolone and an inhibitor of multidrug transporters. PROPOSED COMMERCIAL APPLICATIONS: The project is to develop broad spectrum inhibitors of multidrug transporters providing resistance to fluoroquinolone antibiotics in Gram positive pathogens. Patents will be applied for the improved inhibitors before licensing to a pharmaceutical company. The envisioned commercial product is a combination of such an inhibitor with a fluoroquinolone antibiotic.